The purpose of this project is to understand the role of thyroid hormone-prolactin interactions in the normal growth and differentiation of the mammary gland and the development of mammary tumors. Studies include: 1) examination of development and growth of "spontaneous" mammary tumors in primiparous mice in various thyroid states; 2) evaluation of the role of thyroid hormones in induction of mammary carcinomas in rat by N-nitrosomethylurea; 3) examination of the regulation of prolactin receptors in mammary tissue by thyroid hormones in vivo and in vitro; 4) determination of the effects of altered membrane lipid composition on prolactin binding; 5) characterization of thyroid hormone binding to mammary tissue using biologically active fluorescent thyroid hormone derivatives.